The first episode focuses around several interns from medical school who get placed in a hospital. It's a comedy, and a very good one. But, like Mash, it has very serious undertones. Actually, I can bring a lot of comparasins between Scrubs and Mash.
Excellent show. If I had to describe it, I'd say it's a medical drama parody, but that doesn't really do it justice. It's got a lot of different type of comedy, from straight up slapstick, to cross-episode jokes, to pretty much anything else you could think of. It actually has an active plot, so it's not entirely about the humor, and the more episodes you watch (preferably in order) the better it gets.